Compound magnetic material and electromagnetic interference suppressor

ABSTRACT

A thin electromagnetic interference suppressing body effective for electromagnetic interference suppression at a microwave band. As a material of the electromagnetic interference suppressing body, there is provided a composite magnetic body formed by bonding powder made of a semi-hard magnetic material by an organic bonding agent and having a high magnetic loss at the microwave band. As the semi-hard magnetic material, a metallic magnetic body, such as Fe--Co--Mo alloy, Co--Fe--Nb alloy or Fe--Co--V alloy, or an oxide magnetic body, such as γ-Fe 2  O 3  or Co--Ti substituted Ba ferrite, can be used. Further, as an example of the organic bonding agent, polyurethane resin can be cited.

TECHNICAL FIELD

The present invention relates to an electromagnetic interferencesuppressing body for a high frequency region, particularly, a microwaveband, and in particular, to a composite magnetic material therefor.

BACKGROUND ART

In recent years, the spread of electronic devices, including digitalelectronic devices, using high frequency waves has been advanced and,among them, the spread of mobile communication devices usingquasi-microwave bands or microwave bands has been remarkable. In thosemobile communication devices represented by portable telephones, demandsfor reduction in size and weight are so great that high-density mountingof electronic components has become the greatest technical problem.Accordingly, since the electronic components, the printed wiring, theinter-module wiring, etc. which are mounted overcrowdedly, approach toeach other extremely, and further since the speed-up of the signalprocessing speed is schemed, the inter-line coupling due to theelectrostatic coupling and/or the electromagnetic coupling becomes largeand interference due to radiant noise or the like is caused, so that nota few situations have occurred wherein normal operations of the devicesare impeded.

Conventionally, against such so-called high-frequency electromagneticinterference, a measure has been mainly taken to apply a conductorshield.

However, since the conductor shield is an electromagnetic interferencecountermeasure utilizing reflection of electromagnetic waves due toimpedance mismatch relative to the space, it can provide a shieldingeffect but has a drawback to promote the electromagnetic coupling due toreflection from an undesired radiation source. For solving such adrawback, it could be considered effective as a secondaryelectromagnetic interference countermeasure to use a magnetic loss of amagnetic body, that is, an imaginary part permeability μ" so as tosuppress the undesired radiation.

It has been known that the absorption efficiency of undesired radiationincreases corresponding to a magnitude of μ" in a frequency range whereμ">μ'. Accordingly, for obtaining a large magnetic loss at a microwaveband, it is necessary to realize a characteristic where the real partpermeability attenuates due to the magnetic resonance at a VHF band (30MHz to 300 MHz), a quasi-microwave band (300 MHz to 3 GHz) or alow-frequency side of a microwave band (3 GHz to approximately 10 GHz).

Therefore, it is an object of the present invention to provide acomposite magnetic body where the magnetic resonance appears in a rangefrom VHF band to microwave band and, as a result, a magnetic loss at themicrowave band is large (that is, an imaginary part permeability μ" islarge). It is another object to provide an electromagnetic interferencesuppressing body using such a composite magnetic body.

DISCLOSURE OF THE INVENTION

According to the present invention, there is obtained a compositemagnetic body characterized in that semi-hard magnetic powder is bondedby an organic bonding agent, and a magnetic resonance appears in a rangefrom VHF band to microwave band.

The foregoing semi-hard magnetic powder is characterized in that acoercive force Hc thereof is 10 to 300 Oe.

The foregoing semi-hard magnetic powder is characterized by having aflat or needle shape and being oriented/aligned in the compositemagnetic body.

As a material of the foregoing semi-hard magnetic powder, a magneticmetal alloy, such as Fe--Co--Mo alloy, Co--Fe--Nb alloy or Fe--Co--Valloy, can be cited.

Further, as another material of the foregoing semi-hard magnetic powder,an oxide magnetic body, such as γ-Fe₂ O₃ (maghemite) or Co--Tisubstituted Ba ferrite, can be cited.

According to the present invention, there is obtained an electromagneticinterference suppressing body using, as a material thereof, theforegoing composite magnetic body, and having a large magnetic lose at amicrowave band.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically showing a section of a compositemagnetic body of the present invention;

FIG. 2 is a diagram showing μ-f characteristics of sample 1 and sample 4of a composite magnetic body according to the present invention; and

FIG. 3 is a schematic diagram showing an evaluation system used forcharacteristic evaluation of an electromagnetic interference suppressingbody using a composite magnetic body sample of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, a semi-hard magnetic material with a coerciveforce Hc of 10 to 300 Oe, for example, a metallic magnetic body, such asFe--Co--Mo alloy, Co--Fe--Nb alloy or Fe--Co--V alloy, or an oxidemagnetic body, such as γ-Fe₂ O₃ or Co--Ti substituted Ba ferrite, isused as a raw material.

When the raw material is the metallic magnetic body, coarse powder isobtained by a mechanical milling method, an atomizing method or thelike, then flattened using a wet mill such as an attriter, andthereafter bonded using an organic bonding agent so as to obtain acomposite magnetic body. On the other hand, when the raw material is theoxide magnetic body, flat or needle-shaped fine powder is directlyprepared by a crystallizing method, such as a hydrothermal synthesismethod or the like, and then bonded using an organic bonding agent so asto obtain a composite magnetic body.

For obtaining a large imaginary part permeability μ" in the foregoingcomposite magnetic body, the magnetic powder is flattened orneedle-shaped so as to have a thickness equal to or less than a skindepth, an aspect ratio of the flattened or needle-shaped soft magneticmaterial is set to be approximately not less than 10 for setting adiamagnetic field coefficient Nd to be approximately 1, and the magneticpowder is oriented/aligned in the composite magnetic body. Here, theskin depth δ is given by the following equation.

    δ=(ρ/πμf).sup.1/2

In the foregoing equation, ρ represents resistivity, μ representspermeability and f represents frequency. Here, although values differdepending on target frequencies, it is one of the easiest means, forobtaining desired skin depth and aspect ratio, to specify the meanparticle diameter of the starting material powder when the metallicmagnetic body is used.

As typical milling means which can be used for flattening the metallicmagnetic material, a ball mill, an attriter, a pin mill, etc. can becited. As long as the thickness and the aspect ratio of the magneticpowder satisfying the foregoing conditions can be obtained, there is nolimitation on the milling means.

Further, for ensuring electrical insulation among individual magneticpowder in the composite magnetic body to give a function as anelectrical insulator to the composite magnetic body even in ahigh-density loading state of the magnetic powder, it is preferable toform dielectric layers on the surfaces of the magnetic powder of analloy material. The dielectric layers can be realized as oxide layers ofmetal elements constituting the alloy by oxidizing the surfaces of themetallic magnetic powder. As one example of a means for oxidizing thesurfaces of the metallic powder, it is preferable, in view of easiness,stability and safety of a control, to carry out an oxidizing process bya liquid-phase acid removing method or a gas-phase acid removing methodwhich introduces nitrogen-oxygen mixed gas at a controlled oxygenpartial pressure in a hydrocarbon organic solvent or an atmosphere ofinert gas, particularly for the metallic powder having high activity andrelatively small size.

When the oxide magnetic powder is used as the magnetic powder, it is notnecessary to be processed by the foregoing surface oxidizing processbecause it has high electrical resistance per se.

As an organic bonding agent to be used as one constituent component ofthe composite magnetic body of the present invention, polyester resin,polyethylene resin, polyvinyl chloride resin, polyvinyl butyral resin,polyurethane resin, cellulose resin, ABS resin, nitrile-butadienerubber, stylene-butadiene rubber, epoxy resin, phenol resin, amideresin, imide resin or copolymers thereof can be cited.

There is no particular limitation on the foregoing means forkneading/dispersing the semi-hard magnetic powder and the organicbonding agent to obtain the composite magnetic body, and a preferablemethod may be selected using properties of a bonding agent to be usedand facility of processes as criteria.

As means of orienting/aligning the magnetic particles in thekneaded/dispersed magnetic mixture, there are a method using a shearingstress and a method using a magnetic field orientation, either of whichmay be used.

For explaining a structure of the composite magnetic body of the presentinvention, its section is schematically shown in FIG. 1. Referring tothe same figure, a composite magnetic body 1 is formed by dispersingflat semi-hard magnetic particles 2 into a layer of an organic bondingagent 3 and bonding them. On the other hand, 4 denotes a supportprovided when the strength is required upon handling or when a shieldcharacteristic is required as an electromagnetic interferencesuppressing body in addition to a high-frequency magnetic losscharacteristic. It may be an insulating plate when an improvement inmechanical strength is only aimed. On the other hand, when the shieldcharacteristic is required, it is necessary to consider an electricalcharacteristic and to select a material excellent in electricalconductivity. A copper plate in FIG. 3, which will be described later,is also used as a shield member with high electrical conductivity.

Hereinbelow, more concrete examples will be described.

First, a plurality of ingots of Fe--Cu--Mo alloy, Co--Fe--Nb alloy andFe--Co--V alloy were prepared and coarsely milled by a stamp mill, thensubjected to grinding processes under various conditions using anattriter. Further, while introducing nitrogen-oxygen mixed gas at a 35%oxygen partial pressure, they were agitated for 8 hours in a hydrocarbonorganic solvent and subjected to a liquid-phase acid removing process.Thereafter, a classifying process was carried out to obtain a pluralityof flat magnetic powder samples having different coercive forces Hc. Asa result of analyzing the surfaces of the obtained powder, thegeneration of metal oxides was clearly confirmed so that the presence ofoxide films on the surfaces of the sample powder was confirmed.

On the other hand, γ-Fe₂ O₃ powder and Co--Ti substituted Ba ferritepowder were prepared by the hydrothermal synthesis method so as toobtain oxide magnetic powder samples.

Using these powders, below-described composite magnetic body sampleswere prepared to examine μ-f characteristics.

For the measurement of the μ-f characteristics, the composite magneticbody samples used were processed into toroidal shapes. By inserting itinto a test fixture forming a one-turn coil to measure its impedance, μ'and μ" were derived.

[Sample 1]

A semi-hard magnetic paste was prepared which had the followingcomposition, and formed into a film by a doctor blade process. Afterapplying hot pressing thereto, curing was carried out at 85° C. for 24hours to obtain sample 1.

The obtained sample 1 was analyzed using a scanning electron microscopeand it was confirmed that the particle alignment was in an in-surfacedirection of the sample.

    ______________________________________                                        Flat semi-hard magnetic body                                                                       95 weight parts                                            (Fe--Cu--Mo alloy) Fine powder A                                              Mean particle diameter: .o slashed.20 μm × 0.3 μm.sup.t                                            Coercive force Hc: 25 Oe                     Polyurethane resin 8 weight parts                                             Curing agent (isocyanate compound) 2 weight parts                             Solvent (mixture of 40 weight parts                                           cyclohexanone and toluene)                                                  ______________________________________                                    

[Sample 2]

A semi-hard magnetic paste was prepared which had the followingcomposition, and formed into a film by a doctor blade process. Afterapplying hot pressing thereto, curing was carried out at 85° C. for 24hours to obtain sample 2.

The obtained sample 2 was analyzed using a scanning electron microscopeand it was confirmed that the particle alignment was in an in-surfacedirection of the sample.

    ______________________________________                                        Flat semi-hard magnetic body                                                                       95 weight parts                                            (Co--Fe--Nb alloy) Fine powder B                                              Mean particle diameter: .o slashed.20 μm × 0.3 μm.sup.t                                            Coercive force Hc: 32 Oe                     Polyurethane resin 8 weight parts                                             Curing agent (isocyanate compound) 2 weight parts                             Solvent (mixture of 40 weight parts                                           cyclohexanone and toluene)                                                  ______________________________________                                    

[Sample 3]

A semi-hard magnetic paste was prepared which had the followingcomposition, and formed into a film by a doctor blade process. Afterapplying hot pressing thereto, curing was carried out at 85° C. for 24hours to obtain sample 3.

The obtained sample 3 was analyzed using a scanning electron microscopeand it was confirmed that the particle alignment was in an in-surfacedirection of the sample.

    ______________________________________                                        Flat semi-hard magnetic body                                                                       95 weight parts                                            (Fe--Co--V alloy) Fine powder C                                               Mean particle diameter: .o slashed.30 μm × 0.4 μm.sup.t                                            Coercive force Hc: 130 Oe                    Polyurethane resin 8 weight parts                                             Curing agent (isocyanate compound) 2 weight parts                             Solvent (mixture of 40 weight parts                                           cyclohexanone and toluene)                                                  ______________________________________                                    

[Sample 4]

A semi-hard magnetic paste was prepared which had the followingcomposition, and formed into a film by a doctor blade process. Afterdrying it in a magnetic field in an in-surface direction of a sample,hot pressing was applied thereto and curing was carried out at 85° C.for 24 hours to obtain sample 4.

The obtained sample 4 was analyzed using a vibrating type magnetometerand it was confirmed that a magnetization easy axis was in an in-surfacedirection of the sample.

    ______________________________________                                        Needle-shaped semi-hard                                                                            95 weight parts                                            magnetic body (γ-Fe.sub.2 O.sub.3)                                      Fine powder D                                                                 Mean particle diameter: .o slashed.0.1 × 0.8 μm.sup.t                Coercive force Hc: 270 Oe                                                     Polyurethane resin 8 weight parts                                             Curing agent (isocyanate compound) 2 weight parts                             Solvent (mixture of 40 weight parts                                           cyclohexanone and toluene)                                                  ______________________________________                                    

[Sample 5]

A semi-hard magnetic paste was prepared which had the followingcomposition, and formed into a film by a doctor blade process. Afterdrying it in a magnetic field in an in-surface direction of a sample,hot pressing was applied thereto and curing was carried out at 85° C.for 24 hours to obtain sample 5.

The obtained sample 5 was analyzed using a scanning electron microscopeand it was confirmed that the particle alignment was in a directionperpendicular to an in-surface direction of the sample, and it wasfurther analyzed using a vibrating type magnetometer and it wasconfirmed that a magnetization easy axis was in the in-surface directionof the sample.

    ______________________________________                                        Flat semi-hard magnetic body                                                                       95 weight parts                                            (Co--Ti substituted Ba ferrite)                                               Fine powder E                                                                 Mean particle diameter: .o slashed.1 μm × 0.3 μm.sup.t                                             Coercive force Hc: 295 Oe                    Polyurethane resin 8 weight parts                                             Curing agent (isocyanate compound) 2 weight parts                             Solvent (mixture of 40 weight parts                                           cyclohexanone and toluene)                                                  ______________________________________                                    

Magnetic resonance frequencies fr and imaginary part permeabilities μ"measured with respect to the foregoing respective samples are shown inTable 1 below.

                  TABLE 1                                                         ______________________________________                                                    Magnetic Resonance                                                                         Imaginary Part                                          Frequency fr Permeability μ"                                            ______________________________________                                        Sample 1     80 MHz      4.1                                                    Sample 2  90 MHz 3.5                                                          Sample 3 400 MHz 2.3                                                          Sample 4 850 MHz 1.5                                                          Sample 5 1.1 GHz 1.3                                                        ______________________________________                                    

FIG. 2 shows μ-f characteristics of sample 1 and sample 4. The othersamples exhibited characteristics approximately within these frequencyranges.

As seen from the foregoing Table 1 and FIG. 2, according to the presentinvention, there can be obtained the composite magnetic materials withhigh magnetic losses at the microwave band.

Using the foregoing samples, electromagnetic interference suppressingeffects thereof were measured using an evaluation system as shown inFIG. 3.

Here, a composite magnetic body sample 10 having a thickness of 2 mm anda length of each side being 20 cm was backed with a copper plate 11 soas to prepare an electromagnetic interference suppressing body sample.An electromagnetic wave was radiated to the sample from anelectromagnetic wave generator 12 via a micro-loop antenna 13 having aloop diameter of 1 mm, and a reflected wave from the electromagneticinterference suppressing body sample was received at an antenna 14 ofthe same size and shape so as to measure the strength of the reflectedwave by a network analyzer (electromagnetic field strength measuringdevice) 15.

The results are shown in Table 2 along with the surface resistances.

                  TABLE 2                                                         ______________________________________                                                             Sample 1 Sample 4                                        ______________________________________                                        Surface Resistance (×10.sup.7 Ω)                                                       5.8      4.3                                               Signal Attenuation (dB) (at 8 GHz) 7.9 5.7                                  ______________________________________                                    

Here, the surface resistances are measured values according to theASTM-D-257 method. Values of the electromagnetic interferencesuppressing effect represent signal attenuation when the copper plate isset to a reference (0 dB).

From the foregoing Table 2, the following effects are evident.

Specifically, according to the composite magnetic body of the presentinvention, the value of the surface resistance is 10⁷ to 10⁸ Ω.Therefore, by using the magnetic powder oxidized at least on thesurfaces thereof, the high insulation can be given to the compositemagnetic body so that the surface reflection of the electromagneticwaves due to impedance mismatch as observed with respect to theconductor, the bulk metallic magnetic body or the like, can besuppressed.

Further, it is understood that the composite magnetic body of thepresent invention has the excellent electromagnetic interferencesuppressing effect at the microwave band.

As described above, according to the present invention, there can beobtained the composite magnetic body in the form of the semi-hardmagnetic powder bonded by the organic bonding agent, which has the highmagnetic loss at the microwave band and thus can suppress theelectromagnetic waves at the microwave band. Therefore, using thiscomposite magnetic body, the thin electromagnetic interferencesuppressing body effective at the microwave band can be obtained.

Industrial Applicability

The flexibility can be easily given to the composite magnetic body andthe electromagnetic interference suppressing body of the presentinvention as seen from the constituent components thereof. Thus, it ispossible to deal with a complicated shape or strict requirements forvibration and shock resistance.

We claim:
 1. An electromagnetic interference suppressing bodycharacterized in that a semi-hard magnetic powder is bonded by anddispersed through an organic bonding agent, said magnetic powder beingselected from the group consisting of alloys of Fe--Co--Mo, Co--Fe--Nband Fe--Co--V, said body having a magnetic resonance of VHF band and tomicrowave band.
 2. The electromagnetic interference suppressing body asin claim 1 characterized in that a coercive force Hc of said body rangesfrom about 10 to 300 Oe.
 3. The electromagnetic interference suppressingbody as in claim 1 characterized in that said semi-hard magnetic powderhas a flat or needle shape and is oriented/aligned in said magneticbody.
 4. The electromagnetic interference suppressing body as in claim1, wherein said magnetic powder is Fe--Co--Mo magnetic powder.
 5. Theelectromagnetic interference suppressing body as in claim 1, whereinsaid magnetic powder is Co--Fe--Nb alloy.
 6. The electromagneticinterference suppressing body as in claim 1, wherein said magneticpowder is Fe--Co--V alloy.